Tamper-proof screw having frusto-conical head with flat tool engaging facets thereon



Nov. 20, 1956 R. F. SCHWARTZ 2,

DAMPER-PROOF scREw HAVING FRUS'IO-CONICAL HEAD WITH FLAT T OOL ENGAG FA S THEREON Filed Dec. l9

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United States Patent TAMPER-PROOF SCREW HAVING FRUSTO-CONI- CAL HEAD WITH FLAT TOOL ENGAGING FACETS THEREON Robert F. Schwartz, New York, N. Y.

Application December 15, 1954, Serial No. 475,372

1 Claim. (Cl. 85-45) This invention concerns a novel screw provided with a head shaped in such a manner as to render the screw tamper-proof.

In many applications where screws are used it is necessary that they be so constructed as to be secure against theft, tampering, and similar mischief. Such screws are used extensively in vending machines, parking meters, coin controlled turnstiles, lockers, game devices and so on. While it is permissible that the tamper-proof screws be installed or removed by a specially shaped wrench keyed to the shape of the screw heads, it is necessary to provide that the screws can not be loosened by a conventional screwdriver or knife, a parallel jawed wrench such as an open end or hexagonal wrench, or by a conventional wrench or screwdriver of any type. It is desirable that the screw heads be symmetrical in shape and ornamental in appearance.

The principal object of the invention is therefore directed to providing a screw with a head of novel shape which can be effectively gripped only by a correspondingly shaped wrench and which cannot be turned by any conventional screwdriver or wrench.

It is a further object of the invention to provide a screw having a generally conical head with a plurality of facets spaced around the head.

Other and further objects and advantages of the invention will become apparent from the following description taken together with the drawings, wherein:

Fig. 1 is a top plan view of a screw embodying the invention.

Fig. 2 is an elevational view of the screw of Fig. 1 taken on lines 2-2 of Fig. 1.

Fig. 3 is a sectional view taken on lines 33 of Fig. 1.

Fig. 4 is a top plan view of a modification of the invention.

Fig. 5 is an elevational view of the modification of Fig. 4.

Fig. 6 is a top plan view of another modification of the invention.

Fig. 7 is an elevational view of the modification of Fig. 6.

Fig. 8 is a sectional view taken on lines 88 of Fig. 6.

In Figs. 1, 2 and 3 are shown a screw having a solid shank 10 and a solid head 12. The head is in the form of a conical frustum, that is, it is in the form of a truncated cone with a flat top 13 and a flat bottom integral with shank 10. The conical side of the head of the screw is disposed at an angle of approximately 55 degrees to the base of the head, as clearly illustrated in the drawing. Three facets 14 are formed on the head 12. Each facet 14 extends from the bottom surface 15 of the head part way up the tapered side 19 leaving a major portion of the surface 15 perfectly smooth and unbroken. The facet 14 may be disposed parallel to the axis 0 of shank 10 or slightly inclined thereto as shown by dotted line A. The facet 14 is substantially flat, as if a true conic section had been cut from the head 12 leaving surface 14 with a parabolic side 20 terminating at the bottom surface 15. None of the three facets 14 are parallel to each other and they may be uniformly or non-uniformly spaced around the head. In the structure of the head it will be noted that there are no parallel surfaces which can be gripped by any conventional type of wrench. If the screw is inserted in a suitably threaded hole and tightened by a special wrench having a recess with sides mating with the facets 14, said surface 15 bears on surface S of a support, no conventional wrench can be used to loosen the screw and even hammer blows directed on the facets 14 will be ineffective to loosen the screw. The screw is hardened by heat treating and it cannot, therefore, be removed by drilling it out or filing additional facets thereon to receive a conventional wrench or by any other method.

In the embodiment of Figs. 4 and 5, the three facets 14 are rather concave. One portion 14A of the facet 14' is substantially straight and the remaining portion is curved. Curved portion 14B is at the leading side and portion 14A is at the following side of each facet 14 for a screw tightened in conventional manner by rotation clockwise as shown by arrow T. In this embodiment when the screw is fully threaded and tightened in a threaded screwhole no exposed surface of the head is disposed parallel to any other. The screw is tamperproof because a hammer blow directed to the head on line 17 at curved portion 14B will serve to tighten the screw rather than loosen it; a blow directed on line 18 to straight portion 14A will be ineffective since the line of force will pass within the area of shank 10. A blow in any other direction as on line 16 will glance off the straight portion 14A. No parallel jawed wrench can loosen the screw. If the screw is made with a left hand thread to be tightened by counter-clockwise rotation, the direction of placement of 'facets 14 will be reversed with curved portion 14B leading and portion 14A following in the counter-clockwise direction.

In the embodiment shown in Figs. 6, 7, 8 the head 12 has three scoop-shaped facets or recesses 14". The surface 21 of each recess is rather concave and smooth with its lower edge 21' somewhat inclined to the plane of the bottom surface 15. This embodiment is common with those above described and affords no purchase for effective application of any conventional wrench, screwdriver, or hammer to loosen the screw once it is secured in place.

There has thus been provided a screw having a head so shaped as to be secure againsts loosening by any casual tamperer so that it serves effectively as a tamper-proof element for a vending machine or the like. The ornamental appearance of the various forms of screw heads disclosed is apparent in the drawing.

Although a limited number of embodiments of the invention have been disclosed this has been done only by way of illustration. It will be apparent to those skilled in the art that changes are possible without departing from the spirit of the invention as defined by the scope of the appended claim.

What is claimed and desired to protect by Letters Patent of the United States is:

A substantially tamper-proof screw, comprising a solid threaded shank with a solid conical head integrally formed thereon, said head being generally in the form of a frustum of a cone with a flat top and a flat base, said threaded shank projecting axially from said base, and three nonparallel flat facets formed on said conical side of the head immediately adjacent said base in equally spaced relation to each other and occupying a relatively small portion of the height of the head, said facets being disposed on planes which are parallel to the longitudinal axis of said threaded shank, said facets merging throughout their entire length with the conical side of the head of the screw, said conical side of the head of the screw being disposed at an angle of approximately 55 degrees to the base of the head.

References Cited in the file of this patent UNITED STATES PATENTS Rodd Nov. 20, 1900 Smith Mar. 29, 1904 Rockstad Feb. 17, 1931 H'obler Feb. 20, 1934 Berg Nov. 5, 1940 McCallum May 30, 1950 FOREIGN PATENTS Germany June 29, 1953 

